1. Technical Field
The disclosure relates to an emission control system and an emission control method for an internal combustion engine.
2. Description of Related Art
An emission control system for an internal combustion engine, which includes a catalyst device of a NOx storage reduction type, is known. This type of catalyst device stores nitrogen oxides (NOx) in exhaust gas when burned gas that was burned at a lean air-fuel ratio flows into the device, and releases the stored NOx when burned gas that was burned at a rich air-fuel ratio that is richer than the stoichiometric air-fuel ratio flows into the device. Further, when hydrocarbon (HC) is supplied to this type of catalyst device when NOx is released, the catalyst device reduces and removes the released NOx, using the hydrocarbon as a reductant. Thus, in the emission control system including the catalyst device of the NOx storage reduction type, storage of NOx under lean air-fuel ratios, and release and reduction of NOx under rich air-fuel ratios are alternately repeated, so that emission of NOx to the ambient air is curbed.
In Japanese Patent No. 4893876, an emission control system for an internal combustion engine is disclosed which permits NOx to be subjected to continuous aftertreatment under lean air-fuel ratios, by intermittently adding fuel into exhaust gas for continuous aftertreatment, such that the concentration of HC in exhaust gas flowing into a catalyst device of a NOx storage reduction type oscillates with an amplitude within a specified range and a cycle within a specified range.
In the catalyst device of the NOx storage reduction type, sulfur in exhaust gas is stored along with NOx. Since the sulfur stored in the catalyst device cannot be released under a temperature condition under which NOx is normally released, the amount of sulfur stored in the catalyst device (sulfur storage amount) gradually increases, when the NOx aftertreatment is simply continued by repeating storage and release/reduction of NOx as described above. Then, the sulfur thus stored affects the NOx storage capability of the catalyst device, and induces deterioration of the NOx aftertreatment performance. Therefore, in the emission control system including the catalyst device as described above, when the sulfur storage amount increases to a certain extent, a regeneration control is performed so as to release the stored sulfur and regenerate the catalyst device. The regeneration control is carried out by alternately performing a temperature raising operation to raise the temperature of the catalyst device to a temperature at which sulfur can be released, by burning unburned fuel supplied to exhaust gas through post injection, or the like, in the catalyst device, and a releasing operation to release sulfur by making the air-fuel ratio rich.
Since the temperature at which sulfur can be released is higher than a temperature region in which NOx can be stored, NOx cannot be stored during the temperature raising operation of the regeneration control. On the other hand, continuous aftertreatment of NOx under lean air-fuel ratios through the fuel addition as described above can be conducted in a high temperature region; therefore, when the temperature raising operation is performed through the fuel addition for continuous aftertreatment, the NOx aftertreatment can be continued even during the temperature raising operation.